Imagine that you have a bunch of bombs
for whatever reason And these bombs are
photons-sensitive or light-sensitive
which means that if you shine light or
even a single photon at one of them they
will detonate an explode. But the
problem is that some of these bombs are
duds. Is there a way to test whether
a bomb is live without detonating it? Quantum
physics says there is! These two guys
with really cool names Avshalom Elitzur and
Lev Vaidman, came up with a thought experiment
to solve this problem using what they
called an interaction free measurement,
meaning that they could test whether
a bomb was live without an explosion. To
understand this we need to talk a little
bit about something called quantum
superposition. Quantum superposition is
the theory that an object exists in all
possible states at the same time, and the
act of measuring the object forces it to
choose one state. So there's a very
famous thought experiment about this
called Schrödinger's cat. In a box
there's a cat, a vial of poison,
a hammer, a Geiger counter and some
radioactive material. When the
radioactive material decays the Geiger
counter detects it, triggers the hammer to
smash the vial and kills the cat.
The thing is we don't know when the
radioactive material will decay it's
completely random.
So if we were to guess at any moment in
time would the cat be alive or dead?
Surely it must be one or the other right?
Well according to quantum physics the
can't exist in a superposition of being
both alive and dead. Remember it exists
in all possible states until it is
measured are observed. It is only when we
open the box and take a look that the
cat has to choose a state. Today it
chooses to be alive.
Ok so a cat can't exist in a state of
quantum superposition because it's not a
quantum object, but let's take a look at
another experiment which uses something
called a Mach- Zehnder interferometer. A
Mach-Zehnder interferometer consists
of a light source, a beam splitter, 2
mirrors, another beam splitter and 2
light detectors. The light source emits
a beam of light and the
beamsplitter splits the light exactly
50/50. The mirrors guide the
light toward the second beam splitter,
where the two beams recombined and
interfere.
Now the setup is configured so that the
beams constructively interfere on
detector A and destructively interfere
on detecttor B. I have an entire video
which goes more into constructive and
destructive interference called The
Double Slit Experiment which I've linked
in the description below, but all you
really need to know here is that when
the beams interfere at the second beam
splitter and all of the light will show
up on detector A. Detector B is left
completely dark. So because of the way
the setup is configured, 100%
of the light shows up on
detector A. Cool! What would happen if we
slowed the light beam down? Slowed it down so
much that it only emitted a single
photon at a time.
This is where things get a bit weird.
classically we would expect that 50%
of the time the photon showed up
on detector A, and 50% of the
time it showed up on detector B. I mean
I know we said that the light beams
interfere constructively on A, but you
need two objects to interfere with each
other. So in the case of a single photon
it should show up on both detectors. But
the results showed that 100%
of the time the photon showed up
on detector A. Not a single photon was
detected on B. So what's going on? The
photon interferes with itself? Yeah... So the
photon takes both parhs and interferes
with itself at the second beam splitter.
Now I know what you're thinking. Aren't the
detectros a form of measurement? Well
notice the detectors only really measure
what happens to the photon after it hits
the second beam splitter. If the
photon takes the lower path there's a
50-50 chance it will end up on either
detector A or B, and if the photon takes
the upper path there is still a 50-50
chance it will end up on detector A or
B. So the detectors actually tell us
nothing about which path the photon
took. This not knowing is what allows the
photon to undergo quantum superposition
existing in both possible states and
taking both paths.
I know it sounds crazy but this happens!
So how can we use this weird phenomena
to test bombs?
Well let's first place a dud bomb in the
lower path. Now let's see what happens if
the photon takes the other path. A 50-50
chance it shows up on A or B, ok nothing
unexpected there. Now what if the photon
takes the lower path. There is still a
50-50 chance it shows up on A or B. From
the perspective of the photon,
nothing has changed. It doesn't interact
with the bomb in any way. So the
detectors still tell us nothing about
which path the photon took, meaning
that it still meets the requirements for
quantum superposition. The photon takes
both parhs, interferes with itself and
shows up on detector A. So if the bomb
is a dud, the photon will always 100%
of the time show up on
detector A. Now let's try the live bomb.
Now this is the game changer. We place a
live bomb in the lower path, and the
photon happens to also take the lower
path. The photon sensor is working, the
bomb absorbs the photon and explodes.
So no photon shows up on either detector.
Now if the photon takes the upper path
it will either show up on detector A or
B.
Wait, so the detectors do tell us
something here. If no photon is detected
it took the lower path. If the photon
shows up on either detector it took the
upper path. Because the detectors tell
us which path the photon took, it no longer
meets the requirements for quantum
superposition and is forced to choose a
path. So when the bomb is live 50%
of the time the photon is
absorbed and not detected. 25%
of the time
the photon shows up on detector A and
25% of the time it shows
up on detector B. Now this last
25% is the real
significance of this experiment. So when
no photon is detected the bomb is live.
When a photon is detected on A it could
be either live or a dud and experiment
needs to be repeated.
But when a photon is detected on B the
bomb is live and we were able to detect
this without an explosion.
Ok so there were few explosions. I mean a
25% success rate isn't
the best in the world. But the idea of a
quantum bomb tester is still pretty cool.
What did you think about this thought
experiment? I'd love to hear your thoughts
in the comments below. If you liked this
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so check that out if you like and yeah
I'll see you next time. Bye!